The packaging industry has long sought to develop plastic film, sheet, bottles, wrappings, and other containers which are impervious to oxygen to preserve materials contained therein. That industry has further sought to develop similar items resistant to the passage of carbon dioxide for use in maintaining the carbonation of carbonated beverages. Resistance to passage of water vapor is also important to the packaging industry.
The most useful polymers which exhibit very low values for oxygen permeability are poly(vinylidene chloride) and polymers containing vinyl alcohol, such as ethylene-vinyl alcohol copolymers containing less than about 50 mol percent ethylene units, or homopolymers of hydrolyzed poly(vinyl acetate) known as poly(vinyl alcohol).
Although both types of polymers are utilized in commerce, they have deficiencies which limit their broader use. Poly(vinylidene chloride) is thermally less stable than most polymers and is difficult to process; poly(vinyl alcohol)'s barrier properties are greatly affected by high relative humidity, and the ethylene-vinyl alcohol polymers are not optically clear. Further, the structural properites required for many applications are difficult to achieve with these polymers.
The packaging industry has also sought to prepare containers exhibiting enhanced service temperature for the hot-fill packaging of foods, sterilization prior to packaging, autoclaving to sterilize contents, and the like. Materials attractive for such heat-sensitive uses tend to have poor barrier properties.
It has been known for some time that if laminates are prepared of two or more polymers layers adequately adhered together, the polymer layer or layers having good barrier properties can effectively form a barrier.
The permeability parameter for the multilayered structures can be predicted by use of the following equation: EQU .sup.t TOTAL/P.sub.average =t.sub.1 /P.sub.1 +t.sub.2 /P.sub.2 +T.sub.3 /P.sub.3 +*** +T.sub.n /P.sub.n
where t is the thickness of each individual film, P the known permeability parameter for that polymer, .sup.t total is the thickness of the total composite structure, and .sup.P average is the actual permeability parameter of the composite structure.
This relationship holds only if the films have no defects and if the adhesion is acceptable. If thick tie-layers are required to bond the layer, the thickness and permeability of these tie layers must be included in the equation.
Thermally stable, clear polyglutarimide polymers with improved service temperature as taught by R. M. Kopchik in U.S. Pat. No. 4,246,374 (1981), and reduced polyglutarimide polymers as taught by M. P. Hallden-Abberton et al., U.S. Pat. No. 4,727,117 (1988) are herein incorporated by reference.
The art has described the preparation of composite structures of polyglutarimides with polycarbonates, and has described the general possibility of preparing barrier/non-barrier/barrier tri-layer laminates for packaging use. The art has not described the multilayer structures of this invention.
It is thus an object of the present invention to disclose a multilayer structure having outstanding barrier properties to oxygen comprising layers prepared from one or more polymers having certain desirable physical properties but inadequate gas barrier properties of their own, and a polyglutarimide which has moderately good barrier properties. Another object is to prepare such a barrier composite structure further having excellent optical properties, resistance to impact, and a service temperature sufficient for hot-fill and sterilization. Under appropriate conditions the structure is expected to be an effective barrier for carbon dioxide and water vapor. Also the structure would be an effective barrier against other gases.
Further objects and advantages will be apparent from the following description of the present invention.